There is a general assumption that every individual has a unique fingerprint from which they can be uniquely identified. The skin of an individual's finger forms patterns of ridges and valleys that leave a print on a surface when touched. The print can be sensed by a fingerprint sensor, such as a capacitive sensor, that can generate an image of the pattern of ridges and valleys, in response to the fingerprint sensor being touched to thereby represent the fingerprint electronically in a computer-readable format.
There is a general assumption that every individual has a unique fingerprint from which they can be uniquely identified. The skin of an individual's finger forms patterns of ridges and valleys that leave a print on a surface when touched. The print can be sensed by a fingerprint sensor, such as a capacitive sensor, that can generate an image of the pattern of ridges and valleys, in response to the fingerprint sensor being touched to thereby represent the fingerprint electronically in a computer-readable format.
Authentication of an individual from a fingerprint is the process of comparing at least two instances of fingerprints (skin impressions) acquired by a fingerprint sensor to determine whether these impressions are likely to come from the same individual. As fingerprint sensing and processing technology gradually produce more and more reliable verification and as more advanced processing means become available in electronic devices fingerprint based authentication becomes more widely used—e.g. in connection with access control, authentication, for gaining access to a smart phone. In connection therewith, and in general, there is a demand for very fast acquisition of a fingerprint image and processing thereof. However, for fingerprint sensors—at least those of the capacitive type—it takes a significant amount of time to acquire a fingerprint image.
When it comes to the timing of the acquisition, also denoted capture, then there are basically two things that should be avoided, namely a too early or too late capture. A too early image capture can result in images where the finger is not yet put properly on the touch sensor, while a too late capture can lead to images where the finger is leaving the sensor or where sweat from the finger starts to spread in the valleys. This implies that there is only a limited time interval in which it is feasible to acquire a good fingerprint image.
It should also be noted that the total time it takes until a full fingerprint image is acquired and verification processes are completed is usually an important performance indicator.
US 2015/070137 discloses a fingerprint sensor of the capacitive type which comprises an array of finger sensing pixels and processing circuitry coupled to the array of sensing pixels. The processing circuitry is capable of acquiring finger stability data from a subset of the sensing pixels scattered across the fingerprint sensor, and determining whether a finger is stable relative to the array off sensing pixels based upon finger stability data. The processing circuitry is also capable of acquiring fingerprint images from the array of finger sensing pixels when the finger is determined to be stable.
However, this fingerprint sensor is configured in hardware and provides only limited information for deciding when to acquire a full fingerprint image, which in turn involves the risk of acquiring a fingerprint image that is acquired too early, before the finger is actually stable and before sufficient image information is available or too late, when the finger has left the sensor and the moment for acquiring an image with sufficient image information is missed. In either events re-activation of a user to touch the sensor again are needed, if possible, to obtain sufficient information in the fingerprint image.